PNNL

The Science                                 

The surfaces of materials have a range of compositions, which can change the overall behavior of small particles. The acid-base chemistry of these surfaces is important for applications ranging from catalysis to contaminant adsorption. Boehmite is a key intermediate in aluminum production and a component of industrial wastes, including nuclear waste stored in underground tanks at the Hanford site. Researchers used molecular dynamics (MD) simulations to model reactions between water and the surface of boehmite crystals as a function of pH. They found that the acidity of different boehmite surfaces influences the preferred shape of the boehmite nanoparticles and their ability to adsorb solutes from water.

The Impact

Boehmite is a significant component of legacy nuclear waste contained in tanks. Understanding how it forms and reacts is essential for safe and effective waste processing. This work enhances scientific understanding of how boehmite surfaces behave in different conditions. This has implications for its aggregation behavior and ability to interact with other chemicals in solution. By understanding changes in surface reactivity as a function of solution pH, researchers can better predict the shapes of boehmite nanoparticles and how chemicals will adsorb onto different exposed surfaces.

Summary

Researchers used an integrated classical MD and ab initio MD approach to determine highly accurate values of the acid strength, or pK_a, of all sites on different boehmite surfaces in the presence of water. The connectivity of the hydroxyl, or -OH group, to surface aluminum atoms dominates the pK_a and defines a specific “type” of -OH group. Regardless of the surface location of a specific site, its bond connectivity remains the most important factor in controlling its acidity. However, different boehmite surfaces have a range of hydroxyl populations encompassing three hydroxyl types. For instance, one plane has only a single hydroxyl type, while another has all three. This heterogeneity leads to significantly different surface properties. Researchers determined one of these properties, surface energy, at different pH values to generate theoretical nanoparticle morphologies (shapes). The pK_as were used to predict the point of zero charge for the surfaces, which showed good agreement with prior experimental results reported in the literature.

PNNL Contact

Carolyn Pearce, Pacific Northwest National Laboratory, carolyn.pearce@pnnl.gov

Aurora Clark, University of Utah, aurora.clark@utah.edu

Funding

This research was supported by IDREAM (Interfacial Dynamics in Radioactive Environments and Materials), an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Basic Energy Sciences. The computational work was performed using facilities at the Washington State University Center for Institutional Research Computing, and the Institutional Computing facility at Pacific Northwest National Laboratory.